This invention relates to an apparatus for depositing metal on a substrate by a cathode sputtering technique.
Sputtering refers to the removal of material from a surface due to particle bombardment and is an occurrence commonly observed at the cathode in a D.C. gas discharge. A gas, is admitted into an evacuated chamber containing anode and cathode electrodes. As discussed in U.S. Pat. No. 3,502,562, issued Mar. 24, 1970 to Richard S. Humphries, the positive ions are attracted to and bombard the negatively charged cathode resulting in a liberation of the cathode material. This cathode sputtering results in the disintegrated material leaving the cathode surface either as free atoms or in chemical combination with the residual gas molecules. Some of the liberated atoms may be deposited on the surfaces surrounding the cathode and this effect has been used to produce homogeneous thin films of elements, alloys or compounds on substrates suitably positioned within the evacuated chamber.
The cathode may be suspended from the top of the evacuated chamber but must be connected to a source of high negative potential.
An improved method of sputtering with which we have working involves the use of a magnetron for the vacuum deposition of metals, alloys, compounds and dielectrics. We have found that magnetron sputtering is advantageous over other forms of vaccum deposition, such as evaporation, in view of its ability to produce films of essentially the same composition as the starting material. Furthermore, high rates of deposition can be achieved, which are easily controllable during operation. However, magnetron sputtering requires special fabricated targets configured to suit the particular system. Preparation of such targets requires the target material of the appropriate composition to be in a solid, workable form which is not the case for the targets disclosed in the above-mentioned U.S. patent. Furthermore, it has been found that many materials of interest in vacuum deposition are difficult, if not impossible, to fabricate in suitable forms. One such common example is the Nickel Chromium (Ni-Cr) alloys which find extensive application in thin film technology.
Electrical properties of Ni-Cr over a wide range of alloy ratio (from 0 to 100%) are of interest in thin film resistor fabrication. In particular, films prepared with high Cr content have exhibited remarkable stability of resistance value against temperature variation, a feature that is of great value in microelectronics. But for a few standard alloys such as 80/20, 60/40 and 50/50 available in sheet form, Ni-Cr alloys are not generally easy to obtain or fabricate. As the Cr content increases the alloy becomes progressively hard and brittle, and loses workability by conventional rolling and milling operations.
One suggested technique for obtaining the required film composition is by using a two-gun sputtering method with two separated sputter sources operated simultaneously. In principle with this two-gun method, a substrate located in the region of overlap of emission from the two sources would collect the components in the proper ratio, determined by the operating parameters of the source. In practice, however, it has been found that considerable effort is required to obtain consistency of deposition over useful areas of substrate and to reproduce the same results from run to run.